1. Field of the Invention
The present invention relates to ground or foundation improvement such as accretion of subterranean loose earth, stabilizing incompetent ground, etc. by injection grouting, and to ground investigation. In particular, the present invention relates to a boring-injection device capable of performing ground improvement, a method for improving the ground by means of the device, and also a method for investigating the states of the ground before and after the improvement such as a method for measuring the permeability of the ground site, a method for measuring the strength of the ground site, and a method for measuring mud flush of the ground site, by means of the device.
2. Description of the Prior Art
In injecting a grouting agent for ground or foundation improvement, the so-called 1.5 shot method has been available, which comprises inserting a fluid supplying hollow rod in a borehole, pumping a grouting agent having a gelation time of 1 to 2 minutes into the rod and letting the agent discharge out at the lower end of the rod, thereby infiltrating the agent into the ground (the gelation time will be hereinafter referred to simply as "gel time"). However, the method has such a disadvantage that the agent is liable to run away along an elongated annular gap between the fluid supplying hollow rod and the borehole, so that no effective injection can be obtained. In order to overcome the disadvantage, it may be conceivable to supply a grouting agent of shorter gel time, for example, 1 to 20 seconds, thereby accelerating the gelation and preventing the run away, but the gelation starts earlier in the fluid supplying hollow rod, so that there is a fear of clogging in the rod.
In order to overcome such fear, the so-called 2-shot method has been available, which comprises using a fluid supplying double hollow rod having passages for separately supplying two kinds of grouting agents, mixing the two kinds of grouting agents at the discharge opening of the double hollow rod, thereby preparing a grouting agent mixture of flash setting, and infiltrating the agent into the ground. The runaway of the grouting agent can be substantially prevented thereby. On the other hand, the grouting agent mixture of flash setting is less infiltratable into the ground, and if it is forcedly injected into the ground, the infiltration goes vein-like and a pervaded uniform injection can hardly be obtained. Furthermore, cracks may develop in the ground due to the vein-like infiltration.
In order to solve the problem, an injection method using a grouting agent of short gel time with flash setting and a long gel time grouting agent has been developed, where a fluid supplying double hollow rod as described above is used, and at first two kinds of the grouting agents, which make the flash setting grouting agent when mixed, are supplied separately through the hollow rod inserted to a predetermined depth and mixed with each other before discharging out of the hollow rod. The mixture is discharged laterally from the rod into the annular gap between the hollow rod and borehole to form the so-called packer by the grouting agents, and then the grouting agent of long gel time is supplied through the hollow rod and injected below the packer. Thus, a grouting agent of long gel time can be injected thereby, and consequently a pervaded uniform improvement can be attained around and along the lower portion of hollow rod. However, the formation of the packer by the grouting agents is hard to adjust. Satisfactory packer effects cannot be obtained with too small a discharge amount on one hand, while vein-like infiltration occurs with too large a discharge amount on the other hand and cracks develop in the soil.
Another method capable of injecting a grouting agent of long gel time is a sleeve injection method which comprises inserting an outer pipe into a borehole after boring, filling a cement-bentonite mixture into the gap between the borehole and the outer pipe, inserting an inner pipe into the outer pipe after the curing of the cement-bentonite mixture, the inner pipe having at its lower end two disk or lamp-shade type rubber packers arranged opposedly and provided with a respective periphery in sealing contact with the inner surface of the outer pipe so as to form a tightly sealed chamber between the two packers and the inner surface of the outer pipe, making the tightly sealed chamber meet one group of discharge openings provided stagewise on the outer pipe, supplying a grouting agent into the tightly sealed chamber through the inner pipe, developing cracks in the cement-bentonite by the pressure of the grouting agent discharged from the discharge openings, and injecting the grouting agent into the ground through the cracks, where the injection step is stagewise transferred upwards or downwards to conduct ground improvement in the desired range of depth. The sleeve injection method has such disadvantages that the outer pipe remains inserted and cannot be repeatedly utilized and filling of cement-bentonite is required. Also cracks are neither surely uniformly developed in the cement-bentonite at the injection of the grouting agent, nor artificially adjusted, so that uniform injection of the grouting agent cannot be assured, and furthermore, working steps are increased with much labor.
The present invention also relates to a method for using the device as described at the beginning of the specification.
When the water permeability of soil is to be investigated with respect to the soil layer at various depths at a location to be investigated or measured, in the past, at first a boring machine or a drilling machine with a scaffold for well drilling has been set at a point to be investigated, and the ground has been bored to a desired depth with a rotary boring or impact boring device, while using a mud slurry for protecting the borehole wall from being crumbled, or while inserting an outer pipe (casing) into the borehole for protecting the borehole wall by degrees as the mud slurry boring proceeds. The outer pipe or casing usually has a diameter of 50 mm to 400 mm.
After the mud slurry boring has been carried down to the desired depth and the casing has been inserted into the borehole, a mixture of mud slurry and scrapped soil and sand (slime) filled in the casing is thoroughly replaced with clear water supplied to the casing to wash the casing inside, and is completely washed out of the casing. Then, a pump is inserted into the casing to pump up underground water accumulated in the casing to measure the water permeability of the relevant soil layer, or when there is no room for inserting the water-lifting pump into the casing, the underground water accumulated in the casing is thoroughly removed from the casing by an air lift to measure the water permeability of the soil layer from changes with time in the amount of the underground water accumulated in the casing, or otherwise clear water is introduced into the casing so that a change with time in water level in the casing from the tentatively highest level due to the introduction of the clear water down to the normal water level is observed to determine the water permeability of the soil layer.
Any of these well known methods requires a considerable time from the initial boring of ground to the final measurement with the insertion of a casing, and thus the measurement at many investigation points requires much more time, labor and cost. This means that, when a large area or a long route must be investigated, it has been only possible to make one investigation or one measurement of water permeability per area of 500 to 1,000 m.sup.2 or per route of 100 m or 200 m long. This is also because much time or labor is required for preparatory works in the conventional methods for investigating the water permeability. Excessive auxiliary works are often required for compensating the scarcity of the points in which the investigation have been performed.
In order to measure or investigate the strength proper to soil at various depths at site, the standard penetration test, or the lateral load test that has been recently developed and being now gradually utilized or other various checkup tests depending upon the soil to be investigated are available.
The standard penetration test is a method comprising boring the ground to a predetermined depth by a rotary boring machine, fixing a Raymond soil sampler, 2 inches (about 5 cm) in diameter and about 80 cm long, to the lower end of a boring rod, allowing the weight of 63.5 kg as defined in the standard to fall upon the rod from a height of 75 cm by gravity to plunge the Raymond sampler fixed to the lower end of the rod into the soil to be investigated, and estimating the strength proper to the soil from the required number of hittings until the sampler has been plunged 30 cm deep into the soil (the required number of hittings is generally called "N value").
The lateral load test is a method which comprises boring the ground down to a depth to be investigated by means of a borehole crumbling-preventing pipe, 30 cm in diameter, called "casing" by a rotary boring machine, removing clear water or mud slurry used for boring from the casing after the boring down to the predetermined depth has been completed, inserting an elastomeric cylindrical tube into the casing after the clear water or mud slurry has been removed, inflating the elastomeric cylindrical tube by compressed air to attain tight sealing to the borehole wall, increasing air supply to the elastomeric cylindrical tube to increase the pressure in the tube, and determining a deformation rate of the borehole wall due to the increased pressure from the air supply rate, thereby determining the strength of the soil layer at the desired depth. The elastomeric cylindrical tube usually has an effective length of 1.0 to 2.0 m.
In these standard penetration tests and lateral load tests, boring of the ground must be carried out with the borehole crumbling-prevenging casing before the measurement, and this boring operation takes a large weight on the test work. Particularly, when the soil layer at a depth of, for example, 30 to 50 m or more is to be investigated, a casing of larger diameter must be used, and consequently it takes much time and also much labor in boring. The reliability of the investigation is lowered with increasing depth, and the investigation very often fails to offer correct data that meet the actual state.
In ground boring work using a mud slurry (slurry containing bentonite, slurry containing clay powder or raw clay, or slurry further containing other chemical compounds or natural fibers or the like), flush loss of mud slurry has a great influence upon the progress or work and quality and completion of work, irrespectively of working types.
For example, when the ground includes a layer of large water permeability or voids, the mud slurry used for the boring will continuously run away therethrough, and a considerably large amount of mud slurry as prepared will be ineffectively run away, or the borehole obtained by the boring will be crumbled by the flushing mud slurry and the large amount of flushed mud slurry contaminates the natural underground water to considerably foul the latter. That is, natural environment will be often spoiled thereby. This will be also be true in tunnel working using a mud slurry or mud.
In the operation of boring the ground using a mud slurry, a tendency of mud slurry flush loss has been so far checked up by sampling a large amount of soil at the desired depth at the site by a special means, for example, by a hammer grab, earth drill or other device, stamping the sampled soil in a laboratory and investigating the amount of mud slurry flushed from the stamped soil. In this method, the soil is sampled as disturbed, and thus is far away from the soil proper in its natural state, and only a tendency can be estimated in spite of the expensive, laborious test. There is a great difference in the results between the test directed to the artificially prepared soil and the test of the soil in the natural state. Particularly in the test directed to soil, the structure and state of soil are widely different and usually there are no two of the same structures and states. Any direct mud slurry flush loss test directed to natural soil at any depth at a site (site test) has not been established yet, and only an indirect method for estimating the flushing state from the water permeability of underground water in the ground is now available.